The gasification of biomass for the production of a gaseous input stream for subsequent chemical syntheses is becoming ever more important. One of these syntheses is the Fischer-Tropsch synthesis. The Fischer-Tropsch synthesis is used to produce higher hydrocarbons such as ethylene, propylene, liquid fuels or waxes from an input gas (synthesis gas) containing hydrogen and carbon monoxide.
The achievable yield of higher hydrocarbons such as C5-20 hydrocarbons and lighter hydrocarbons such as C2-4 hydrocarbons as input materials for the chemical industry is substantially determined by the necessary discharge of CO2 to increase the H2/CO ratio in the synthesis gas, the energy requirement and the achieved conversion rates and selectivities in the individual process stages. These separation tasks are as a rule expensive in terms of energy and equipment, and have a negative effect on the commercial implementation of biomass gasification on a larger scale.
In addition to hydrogen and carbon monoxide, a synthesis gas produced by biomass gasification also contains at least methane, carbon dioxide and water. The adjustment of the ratio of carbon monoxide and hydrogen in the input stream takes place according to the state of the art via methane reforming, water-gas shift reactions and carbon dioxide separation. Furthermore, a water separation is necessary (M. J. A. Tijmensen et al., Biomass and Bioenergy 23 (2002) 129-152). The reaction processes are expensive in terms of equipment. The same is true for gas drying and carbon dioxide separation processes. These are realized by physical or chemical absorption processes. Furthermore, a disadvantage in these processes is the use of a separation aid which has to be treated in an energy-intensive way by an additional process step.
Unreacted hydrogen is advantageously fed back into the reactor. For this, the hydrogen can be separated off by means of pressure swing adsorption processes. A disadvantage in both processes is the necessity to cool the gas before entry into the separation unit.
The products can be separated off e.g. by low-temperature rectification at increased pressures. Again, the adjustment of these operating conditions is expensive in terms of equipment and energy, above all if light gases such as hydrogen, methane or carbon monoxide are still located in the product gas to be separated off.
Any carbon dioxide present must be removed before entry into the low-temperature rectification. Absorption processes using detergents with chemical or physical action are state of the art for this.
CN 101 186 550 A describes a process in which two Fischer-Tropsch reactors are operated in series. The product streams are separated in gas-liquid or gas-liquid-liquid separators. The product streams of these separators that contain the light gases are admixed with the input streams of the Fischer-Tropsch reactors. No adjustment of the synthesis gas ratio is described.
CN 101 307 245 A discloses a fixed-bed reactor for Fischer-Tropsch synthesis. The recirculation of unreacted input gases is also discussed. The process used for this is not disclosed.
CN 101 434 507A discloses a cascade consisting of condensers and subsequent separating tanks in which the Fischer-Tropsch products are separated. For this, the product gas is cooled, with the result that a liquid phase forms, which is separated off. The gas remaining after the cooling is cooled further, with the result that a liquid phase to be drawn off forms again. This procedure is repeated until only hydrogen and carbon monoxide remain contained in the gas, which is then recirculated. The liquid phases that form are mixed and fed to a demethanizer column. The process relates to the utilization of vapour-liquid phase equilibria.
CN 101 559 320 A discloses a process in which the gases produced in the reactor are cooled and fed to a demethanizer column. The hydrocarbon mixture discharged in the bottom of the column is fed to a further treatment. The top product is fed to an absorption stage in which the higher hydrocarbons still contained are separated with the help of an absorption liquid consisting of higher hydrocarbons and fed back to the inlet of the demethanizer column. The gases exiting the absorption column at the top are discharged. Membrane processes are not used.
CN 101 979 468 A discloses the recirculation of the unreacted gas from a Fischer-Tropsch reaction, the mixing with carbon dioxide and the subsequent catalytic reforming. This gas is fed, together with the synthesis gas, to the Fischer-Tropsch reactor. The adjustment of the synthesis gas ratio is achieved by the mentioned reforming.
DE 10 2010 011 076 A1 discloses a process in which a medium-boiling product fraction is to be obtained from a Fischer-Tropsch reactor. For this, the high-boiling hydrocarbons are first removed into two gas-liquid separators connected one behind the other and fed back to the reactor via a heated line. In the second separator, the liquid phase is discharged from the process as product and water is removed from it in a liquid-liquid separator. The gaseous product stream is again fed to the reactor.
U.S. Pat. No. 4,049,741 discloses a process in which the octane number of the Fischer-Tropsch products is increased by further reaction steps. Gas streams are separated and fed back into the reaction part.
WO 03/072530 A1 proposes an improved treatment and partial recirculation of Fischer-Tropsch products. The recirculated streams partially undergo a chemical conversion.
WO 01/42175 A1 discloses a process in which a synthesis gas is produced from carbon monoxide, hydrogen and carbon dioxide and fed to a Fischer-Tropsch reactor. The product gas from the reactor is divided into a higher hydrocarbon fraction for further treatment, an aqueous phase and an exhaust-gas stream to be recirculated to the reactor inlet. The exhaust-gas stream is fed to a steam reforming again before being admixed with the reactor input stream. Pressure swing adsorption is mentioned as a method for separating off hydrogen. The reverse water-gas shift reaction is mentioned for the adjustment of the carbon dioxide content.
WO 2004/092306 A1 discloses a process in which unreacted input gases are also recirculated. The separation of the gases takes place by pressure swing adsorption.
The recirculation of unreacted hydrogen and carbon monoxide by means of pressure swing adsorption is also proposed in FR 2 807 027 and FR 2 891 277.
WO 2005/005576 A1 discloses removing carbon dioxide from the exhaust gas forming during the Fischer-Tropsch synthesis and feeding said exhaust gas to an energy recovery.
There is a need to further improve the production of higher hydrocarbons by means of Fischer-Tropsch synthesis from synthesis gas which can be obtained for example from biomass gasification or from biogas, in particular from an energy and equipment point of view.